Semiconductor dies are widely used for a large number of applications, such as microcontrollers, memory elements and other integrated circuits (ICs). Especially in portable devices, size has become an issue despite the minimal dimensions of these dies. When a large number of memory units and other elements are required in a device, both the covered area as well as the interconnection between several dies is a problem. Therefore, solutions are developed which include three-dimensional die architectures, i.e. a number of dies is stacked on top of each other within a single chip package, also known as 3D ICs. This technology has significant advantages including increased packaging density, increased number of possible connections between the dies, and reduced energy consumption due to lower interconnect capacitances.
Operation of any silicon die produces heat as a side effect. Since heat may damage or even destroy a semiconductor die, many dies include a thermal sensor which allows for determining the current temperature of a die. Depending on the temperature, die operation may be controlled in a way that prevents damaging temperatures. In a simple implementation, use of the complete device is prevented if one of its parts is overheating. In other implementations, usage of an overheating part may be throttled, increasing response time of the element but otherwise continuing operation of the device. An example is to speed up the refresh cycle of DRAM (Dynamic Random Access Memory) memory elements, which is necessary for data retention at higher temperatures but also decreases throughput of the device.
There are several issues in both single die and stacked die environments which are not considered in known solutions. One temperature sensor on a die may not be sufficient for an effective control. The temperature on a die may vary between different areas of the die. This may be due to a more extensive usage of certain areas, such as more frequently used memory banks and rows, but also due to constructional features. A single temperature sensor on a die, as often used, may therefore not give sufficient information on the actual heat source. Even worse, a single sensor which is located at some distance from the heated spot will discover overheating later than actually occurred, which means that safety margins have to be added to the temperature thresholds for the sensor.
Especially in a stacked or three-dimensional structure, where a die may be sandwiched in between further dies, heat conduction of the substrate materials may lead to heating of adjacent dies. That is, not only the actual heat source will show a higher temperature, but also dies below or above may give higher temperature readings. Since the reaction to increased temperatures is usually a deactivation or throttling of die usage as described above, this may lead to a decreased performance of several dies or die areas which are not the real source of overheating. Another reason to determine the heat source as exact as possible is that large temperature differences within a die stack should be avoided. Excessive temperature differences may warp the die stack and as a consequence cause fatigue and potential material failures.
Nevertheless, the current approach for thermal sensors cannot simply be extended to multiple sensors, which may help to determine an exact location of a heat source. The logic required for multiple sensing on a die would be significant. A larger number of analog to digital converters (ADCs) and possibly other control logic would be necessary, which would lead to increased cost. Similarly, multiple sensors would linearly increase the required number of connection pins.